We propose a joint theoretical and observational project aimed to probe the link between binary central stars of planetary nebulae (PNe) and the chemical composition and kinematics of their gaseous envelopes. We will extend our pioneering work and continue with our surveys to find new central binary systems. This will eventually provide the still missing distribution of orbital periods and the dynamical characterisation of the binaries, a primordial step to link the fundamental parameters of these systems to those of their nebulae. We want to dig deeper into several open questions regarding the chemistry of the PN gas, that may be related to the presence of a binary nucleus in the core of the nebulae:

i) the connection between large "abundance discrepancies" with the presence of a post-common-envelope binary system;

ii) the influence of binary interactions on the production of heavy elements by slow neutron capture (s-elements) in the asymptotic giant branch phase. This project will gather together leading researchers in both the dynamical characterisation of binary stars within PNe and the study of the chemistry and physics of the interstellar medium. For chemical abundances and kinematics of PNe we will carry out intensive observing campaigns using state-of-the-art instruments on large aperture telescopes such as UVES, FLAMES and MUSE@VLT, and MEGARA and EMIR@GTC. The data will be compared with detailed 3D photoionisation models that we will develop using the MOCASSIN photoionisation code. We will also accurately measure the fundamental parameters of the central binary stars through simultaneous modelling of light curves in different photometric bands and radial velocity curves of both components of the system. For this task, we will use an advanced Wilson-Devinney code such as PHOEBE. This project has fundamental implications for several fields of stellar evolution, such as the physics of mass loss in evolved stars, the nature and evolution of the interacting binaries (including the formation of type-Ia supernovae), the formation of stellar jets, as well as the chemical enrichment of the Universe.